The present disclosure relates, in various exemplary embodiments, to the protection of documents using a top protective photochromic switchable layer.
Photochromic self-erasing, reimageable paper is written by using UV light of 365 nm wavelength. The printed information is readable for a predetermined period of time, for example 4 hours. After passage of a longer time period, for example 20 hours, the printed information is self-erasing, leaving a blank document ready to be reimaged with new information. These documents are often referred to as temporary or transient documents.
Because UV light is used both for writing, as well as for reading (UV light is typically present in room-light from a bulb or sunlight), the unimaged areas of the document are sensitive to the UV component from the reading light. And so, unimaged areas become colored after a period of time, thereby reducing the contrast between the white and colored states, and thus reducing the readability of the document. This is illustrated schematically in FIG. 1, wherein contrast degradation of temporary documents under ambient light is depicted. As the document is continually exposed to ambient light or sunlight, the UV component of light causes a gradual, uncontrolled writing on the document background areas, causing contrast loss.
This problem has previously been addressed by creating a band-pass window for the incident light capable of isomerising (i.e. inducing coloration) the material, centered around 365 nm. The stability of such transient documents has been significantly improved, when compared to unprotected documents. However, the unimaged areas of the documents are still sensitive to the UV component of visible light, centered at 365 nm. It would be beneficial if the transient document were protected against ambient UV light over the entire UV region.